1. Field of the Invention
The present invention relates to a pressure regulating valve for proportionally controlling an output fluid pressure with respect to an input fluid pressure by utilizing an external fluid pressure source and more particularly to a pressure regulating valve fit for use in a fluid pressure brake which is designed to improve responsivity as well as operating feeling at the time the brake is applied.
2. Description of the Prior Art
In a known fluid pressure brake system of the sort described in Japanese Patent Publication No. Sho. 61-53265, for example, a fluid pressure is supplied from a fluid pressure pump to a brake while the fluid pressure is regulated by a pressure regulating valve.
A brief description will subsequently be given of the fluid pressure brake system described in the aforesaid Japanese Patent Publication by referring to FIG. 5. In this system, an oil pressure is generated in a master cylinder 302 when a driver puts on a brake pedal 301 and the pressure is led to a modulator cylinder 303 via branch pipe lines 304a, 306a and acts on the bases of pistons 307, 308, thus causing the pistons 307, 308 to be displaced to the left. Then the conical end portion 310 of an isolation valve 309 causes the hole 311 of the piston 308 and a flow channel 312 to close, so that a servo oil pressure is generated in a chamber 313.
The fluid pressure generated in the chamber 313 causes the isolation valve 309 to be displaced against the force of a reset spring 314 and this results in making working fluid flow into a distributing chamber 315 and further into each chamber 317 of both servo cylinders via a pipe line 316. The pressure generated in the chamber 317 causes a servo piston 318 to be displaced and a stem 319 is displaced to the right in FIG. 5 in this state. Consequently, a flange 330 in the end portion of each stem engages with a corresponding auxiliary piston 321, so that the hole 322 of each auxiliary piston 321 is closed.
Then the working fluid in the chamber 323 is pressurized by the thrust of the servo piston 318 and transmitted from the master cylinder 302 so as to increase oil pressure existing in the chamber 323 proportionally further. On receiving the servo force, the auxiliary piston 321 operates the brake with the effect of applying the brake when the brake pedal 301 is stepped on lightly.
When the fluid pressure in the master cylinder is held to be constant after the brake is operated, however, the discharge pressure of the pump also has to be held constant in the aforesaid fluid pressure brake system which employs the pressure regulating valve comprising the pistons 307, 308 and the isolation valve 309. For this reason, the load of a motor for driving the pump is not released and a great deal of electric power has been consumed during the time the fluid pressure is thus held. Further, the responsivity and feeling characteristics have been insufficient because of the absence of jump-up characteristics in the initial state of the application of the brake.